The present invention relates to a percussive down-the-hole hammer for rock drilling, and a method for adjusting air pressure.
A prior art drill bit for a down-the-hole (DTH) hammer is disclosed in Beccu et al. U.S. Pat. No. 6,062,322. The drill bit comprises an extended anvil portion on which a piston impacts repeatedly to advance the down-the-hole hammer through the rock. The problem that this invention addresses is one of maximizing DTH hammer performance for a given air compressor. In blasthole or deep hole applications, a drill rig will have an air compressor rated for a given air flow at a given maximum pressure. A DTH hammer acts like an orifice (i.e., a restriction) at the end of the drill string. The more air volume (cubic feet per minute) applied, the higher the operating pressure. If this orifice (hammer) is too big, the pressure will not build up high enough to operate at maximum efficiency. Conversely, if this orifice is too small, the compressor will overpressure and cause problems at the compressor. Since an air compressor is roughly 10 times the dollar value of a DTH hammer, it makes sense to design the hammer to the compressor size. There are many different compressor sizes on the market today. A few examples are listed below with the rated flow in cubic feet per minute followed by the rated pressure in pounds per square inch:
750/350
750/250
450/250
840/350
650/350
900/350
1000/350
1050/350
A current waterwell hammer as disclosed in U.S. Pat. No. 6,062,322 would hold approximately 350 psi (pounds per square inch) pressure with the 1050/350 compressor. It will hold approximately 340 psi with the 1000/1350 compressor. It will hold approximately 300 psi with the 900/350 compressor and approximately 280 psi with the 840/350 compressor. The ideal range to operate the hammer is in the 320 psi to 340 psi pressure range. If a compressor is delivering 900 cfm (cubic feet per minute), the pressure will never be in the ideal range for that kind of hammer. A simple and inexpensive adjustment needs to be made to allow the hammer to run at a higher pressure. There are a number of ways to increase or decrease the hammer air consumption for a given hammer.
1. Changing the orifice size that feeds the top chamber
2. Changing the top chamber size
3. Changing the piston stroke
4. Some combination of the above
Prior hammers have used alternative No. 4 to decrease the air consumption on a low volume hammer by shortening the stroke, reducing the size of the top chamber, and restricting the orifice feeding the hammer. The problem there is one of cost. Sometimes the customer would have to buy a completely new hammer.
One object of the present invention is to provide an efficient down-the-hole hammer that is superior to prior art hammers.
Another object is to provide a down-the-hole hammer that allows simple and inexpensive adjustability.
Still another object is to provide an easy method to adjust the air pressure in a down-the-hole hammer to match the compressor output.
A first aspect of the present invention relates to a down-the-hole percussive drill for rock drilling. The drill comprises a generally cylindrical casing adapted to carry a drill bit. A piston is mounted in the casing for reciprocation in a longitudinal direction to repeatedly impart impacts to the drill bit. A hollow top sub is mounted at a rear portion of the casing. The top sub comprises a front face facing toward the piston. A hollow feed tube is mounted to the top sub and extends forwardly along a longitudinal center axis of the casing and defines a center passage adapted to conduct pressurized air. The piston includes an axial through-hole slidably receiving the feed tube, wherein the piston is slidable in a space disposed in front of the front face. The front face and the feed tube together form a recess which opens toward the piston. A volume-changer is removably insertable into the recess to vary a volume of the space and a pressure at which the piston operates.
Another aspect of the invention relates to the top sub per se.
Another aspect of the invention relates to a method of utilizing the above-described apparatus for varying a pressure at which the hammer operates.